UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No.
_
)*

IDEX CORPORATION

(Name of Issuer)

Common Stock

(Title of Class of Securities)

45167R104

(CUSIP Number)

December 29, 2017

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule
pursuant to which this Schedule is filed:

[X]

Rule 13d-1(b)

[ ]

Rule 13d-1(c)

[ ]

Rule 13d-1(d)

*
The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities,
and for any subsequent amendment containing information which would alter the
disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be
deemed to be "filed" for the purpose of Section 18 of the Securities Exchange
Act of 1934 (the "Act") or otherwise subject to the liabilities of that section
of the Act, but shall be subject to all other provisions of the Act (however,
see the Notes.)

Provide the following information regarding the aggregate number and
percentage of the class of securities of the issuer identified in Item 1.

(a)

Amount Beneficially Owned:

See the responses to Item 9 on the attached cover pages.

(b)

Percent of Class:

See the responses to Item 11 on the attached cover pages.

(c)

Number of shares as to which such person has:

(i)

sole power to vote or to direct the vote

0

(ii)

shared power to vote or to direct the vote

See the responses to Item 6 on the attached cover pages.

(iii)

sole power to dispose or to direct the disposition of

0

(iv)

shared power to dispose or to direct the disposition of

See the responses to Item 8 on the attached cover pages.

Item 5.

Ownership of Five Percent or Less of Class.

If this statement is being filed to report the fact that
as of the date hereof the reporting person has ceased to
be the beneficial owner of more than five percent of the
class of securities, check the following:
[ ]

Item 6.

Ownership of More than Five Percent on Behalf of Another Person.

The securities as to which this Schedule is filed
are owned of record by clients of one or more investment advisers identified
in Exhibit A directly or indirectly owned by
Wellington Management Group LLP.
Those clients have the right to receive, or the power to direct the
receipt of, dividends from, or the proceeds from the sale of, such
securities. No such client is known to have such right or
power with respect to more than five percent of this class of
securities, except as follows:

Not Applicable.

Item 7.

Identification and Classification of the Subsidiary Which Acquired
the Security Being Reported on by the Parent Holding Company.

See attached Exhibit A.

Item 8.

Identification and Classification of Members of the Group.

Not Applicable.

Item 9.

Notice of Dissolution of Group.

Not Applicable.

Item 10.

Certification.

By signing below I certify that, to the best
of my knowledge and belief, the securities
referred to above
were acquired and are held
in the ordinary course of business and
were not acquired and are not held for the purpose
of or with the effect of changing or
influencing the control of the issuer of the
securities and were not acquired and are not held
in connection with or as a participant in any
transaction having that purpose or effect.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief,
I certify that the information set forth in this statement is true,
complete and correct.

By: Wellington Management Group LLP

By: /s/ Silvestre A. Fontes

Name: Silvestre A. Fontes

Title: Authorized Signor

Date:
February
14,
2018

By:
Wellington Group Holdings LLP

By: /s/
Silvestre A. Fontes

Name:
Silvestre A. Fontes

Title:
Authorized Signor

Date:
February
14,
2018

By:
Wellington Investment Advisors Holdings LLP

By: /s/
Silvestre A. Fontes

Name:
Silvestre A. Fontes

Title:
Authorized Signor

Date:
February
14,
2018

By:
Wellington Management Company LLP

By: /s/
Silvestre A. Fontes

Name:
Silvestre A. Fontes

Title:
Authorized Signor

Date:
February
14,
2018

Exhibit A

Pursuant to the instructions in Item 7 of Schedule 13G,
the following lists the identity and Item 3 classification
of each relevant entity that beneficially owns shares of the
security class being reported on this Schedule 13G.

Wellington Group Holdings LLP – HC

Wellington Investment Advisors LLP – HC

Wellington Management Global Holdings, Ltd. - HC

One or more of the following investment advisers (the “Wellington Investment Advisers”):

Wellington Management Company LLP – IA

Wellington Management Canada LLC – IA

Wellington Management Singapore Pte Ltd – IA

Wellington Management Hong Kong Ltd – IA

Wellington Management International Ltd – IA

Wellington Management Japan Pte Ltd – IA

Wellington Management Australia Pty Ltd - IA

The securities as to which this Schedule is filed by
Wellington Management Group LLP,
as parent holding company of certain holding companies and
the Wellington Investment Advisers, are owned of record by
clients of the Wellington Investment Advisers.
Wellington Investment Advisors Holdings LLP controls directly,
or indirectly through Wellington Management Global Holdings, Ltd.,
the Wellington Investment Advisers.
Wellington Investment Advisors Holdings LLP is owned by
Wellington Group Holdings LLP. Wellington Group Holdings LLP
is owned by Wellington Management Group LLP.

EXHIBIT B

JOINT FILING AGREEMENT

The undersigned hereby agree that this Schedule 13G (the “Schedule 13G”)
with respect to the common stock of IDEX CORPORATION is,
and any additional
amendment thereto signed by each of the undersigned shall be, filed on
behalf of each undersigned pursuant to and in accordance with the
provisions of 13d-1(k) under the Securities Exchange Act of 1934,
as amended, and that all subsequent amendments to the Schedule 13G
shall be filed on behalf of each of the undersigned without the necessity
of filing additional joint filing agreements. The undersigned acknowledge
that each shall be responsible for the timely filing of such amendments,
and for the completeness and accuracy of the information concerning it
contained therein, but shall not be responsible for the completeness and
accuracy of the information concerning the other, except to the extent that
it knows or has reason to believe that such information is inaccurate. It is
understood and agreed that the joint filing of the Schedule 13G shall not be
construed as an admission that the persons named herein constitute a group for
purposes of Regulation 13D-G of the Securities Exchange Act of 1934, nor is a
joint venture for purposes of the Investment Company Act of 1940.